270 VEGETABLE PHYSIOLOGY 
tion of prussic acid and benzoic aldehyde. Some plants 
have been shown to be capable of utilising the former of 
these, toxic as it is to many forms of animal life. 
The bye-products include bodies of very varying degrees 
of complexity, some nitrogenous and others not. Among 
the former may be mentioned the great group of the 
alkaloids, many of which have not so far been found able 
to minister to the nutrition or growth of the plant, though 
their nitrogen is in organic combination. If a plant is 
supplied with them, but with no other form of combined 
nitrogen, it is rapidly starved. In certain cases in which 
relatively large quantities of them are stored in seeds, they 
have been observed to diminish in amount during germi- 
nation. They may have a nutritive value in these cases. 
Many physiologists consider this group to belong rather to 
the definite excretions of the plant than even to its bye- 
products. They are usually deposited in regions which are 
situated well away from the seats of active life, such as the 
bark of trees, the pericarps of fruits, &c. It is apparently 
very difficult to draw a distinct line of separation between 
excretions and bye-products, just as it is to distinguish 
clearly between the latter and secretions. , 
The amidated fatty acids, as we have seen, generally 
occur in direct relation to nutrition. We have examined 
the part played by leucin and asparagin in protein con- 
struction and metabolism. Several other related substances 
are met with in various plants, but how far they are avail- 
able for nutrition and how far they are merely bye-products 
is uncertain. Such substances are xanthin and glycin, 
which can be extracted from various cells. The latex of 
plants frequently contains many of these substances. 
Caoutchouc is also a frequent constituent of latex. 
Among the non-nitrogenous bye-products may be men- 
tioned the great variety of vegetable acids. Conspicuous 
among these are tartaric, malic, citric, and acetic acids. 
They are usually regarded as arising in the course of the 
katabolic processes, but it is at least possible that some of 
